Natural-Color Pink, Purple, Red, and Brown Diamonds

FEATURE ARTICLES

NATURAL -C OLOR PINK , P URPLE , RED , AND BROWN DIAMONDS : BAND OF MANY COLORS Sally Eaton-Magaña, Troy Ardon, Karen V. Smit, Christopher M. Breeding, and James E. Shigley

Among fancy-color diamonds, naturally pink to red stones are among the rarest and most valuable. The color origin for natural pink diamonds is due to either absorption from the as-yet-unidentified “550 nm” defect(s) that are correlated with shear stress and natural plastic deformation or, very rarely, the NV 0/– centers (nitrogen combined with a vacancy). In this article we summarize prior research as well as the physical characteristics of more than 90,000 natural pink, orangy pink, red, purple, and brown diamonds from the internal GIA database. (Browns and purples are included here, as they coexist on a color continuum with pink diamonds.) We also present absorption and luminescence spectra on a representative subset of 1,000 diamonds to provide an un - paralleled gemological and spectroscopic description of these rare gemstones. For pink diamonds, the gemo - logical observations closely correlate with diamond type and the extent of nitrogen aggregation (i.e., type IaAB, and type IIa pinks show distinct characteristics). Since type IaA

iamond is one of Earth’s most extraordinary ish pink to pinkish brown, pink, red, purple, and un - materials. It represents the pinnacle for sev - modified brown diamonds. Although this article cov - Deral material and physical properties. As a gem, however, it is the near-perfect examples—dia - monds attaining the D-Flawless distinction—and those with imperfections resulting in a vibrant or surprising color that create the most enduring im - In Brief pressions. Fancy-color natural diamonds are among • Almost all pink/purple/red/orangy pink diamonds owe the most highly valued gemstones due to their at - their color to the presence of an as-yet-unidentified tractiveness and great rarity. The 18.96 ct Winston 550 nm absorption band. Minor differences in band Pink Legacy, with a color grade of Fancy Vivid pink, characteristics along with the presence of other defects recently made history by selling at over $50 million, (such as N3 and H3) lead to differences in hue and its $2.6 million per carat price an all-time high for a saturation. pink diamond (Christie’s, 2018). • Among “pink” diamonds colored by the 550 nm band, This article is the third in a series of studies on distinctions in color distribution and observed strain can be correlated with their diamond type. natural colored diamonds: Breeding et al. (2018) dis - cussed the color origin of green diamonds, and Eaton- • A sizable percentage of the “pink” diamonds studied, Magaña et al. (2018) dealt with blue/gray/violet particularly those with saturated colors, are consistent with material found at the Argyle mine, scheduled to diamonds. Here we summarize the color origin of close in 2020. orangy pink, purplish pink to pinkish purple, brown -

See end of article for About the Authors and Acknowledgments. ers a wide variety of color descriptions compared to, GEMS & G EMOLOGY , Vol. 54, No. 4, pp. 352 –377, http://dx.doi.org/10.5741/GEMS.54.2.352 for example, the first article in the series, which was © 2018 Gemological Institute of America limited to natural diamonds with only green as the

352 NATURAL -C OLOR PINK DIAMONDS GEMS & G EMOLOGY WINTER 2018 Figure 1. The 34.65 ct Princie (center, photo courtesy of Christie’s) is a famous example of a pink diamond colored by NV 0/– centers. It is flanked by an array of natural brown, brown-pink, orangy pink, purple-pink, and pink dia - monds with the far more common cause of color—the 550 nm absorption band. These examples span the diverse color range of pinks submitted to GIA. The photos in this compilation are not imaged to scale.

dominant description, the color descriptions are ral “pink” diamonds, principally colored by an as-yet- joined here, as the vast majority of them (which will unidentified “550 nm” absorption band, in order to be quantified below) contain the 550 nm band as a help the trade better understand how these beautiful prevalent absorption feature. For the sake of simplic - diamonds with extremely valuable colors originate. ity, this broad range of color descriptions (e.g., figures 1 and 2) will generally be shortened to “pink” in this CAUSES OF COLOR article. All colored diamonds submitted to GIA are subject Previously, several aspects of gem-quality “pink” to a variety of spectroscopic analyses. The analytical diamonds have been addressed, such as their gemo - techniques and specific details of the instrumenta - logical characteristics (e.g., King et al., 2002); their tion and methods used are summarized in Breeding defects and associated spectroscopic features (e.g., et al. (2018; see supplementary table S-1 at Titkov et al., 2008; Deljanin et al., 2008; Fisher et al., https://www.gia.edu/gems-gemology/spring-2018- 2009; Gaillou et al., 2010; Stepanov et al., 2011; natural-color-green-diamonds-beautiful-conundrum). Byrne et al., 2012a,b; Titkov et al., 2012; Gaillou et “Pink” color in diamonds spans a wide range in al., 2012; Howell et al., 2015); and reports on notable the GIA color description terminology (figure 2; King “pink” diamonds (Smith and Bosshart, 2002; King et et al., 1994; King, 2006). In this article, we include all al., 2014). Here we review the published literature natural fancy-color diamonds with pink, purple, or and supplement it by compiling results obtained red as the dominant hue (i.e., the final name in the from 90,000+ natural diamonds recorded in the GIA color description) in addition to unmodified brown. internal database. Diamonds with yellow-brown or orange-brown color Our purpose is to provide a detailed account of the will be discussed in an upcoming article. In the full gemological and spectroscopic characteristics of natu - dataset of 90,000+ “pink” diamonds seen at GIA be -

NATURAL -C OLOR PINK DIAMONDS GEMS & G EMOLOGY WINTER 2018 353 Increasing Saturation

Very Light pink Light pink Fancy Light pink Fancy pink Fancy Intense pink Fancy Deep pink Fancy red

Increasing Brown Increasing Orange

Fancy brown Fancy pink-brown Fancy brown-pink Fancy brownish Fancy Intense orangy Fancy Vivid pinkish orangy pink pink orange

Increasing Purple

Fancy Vivid purplish pink Fancy Intense purple- Fancy pink purple Fancy Vivid pink purple Fancy Vivid purple pink

Figure 2. These examples of diamonds with varying hues and color saturations represent the “pink” color ranges. Photos by GIA staff. The photos in this compilation are not imaged to scale.

tween January 2008 and December 2016, the highest Various colors and representative visible-near infrared percentage were unmodified pink (40%), followed by spectra are shown in figures 2 and 4, respectively. pinkish purple to purplish pink (28%), brownish pink to pinkish brown (17%), and orangy pink (10%); see Colored Deformation Lamellae. In many brown, the color distribution in figure 3. The rarest colors pink, and purple diamonds, the color is concentrated were unmodified brown (3%); purple with brown or within parallel narrow bands (referred to as “brown gray modifiers (1%); unmodified red (0.5%); red with graining,” “pink graining,” or glide planes, or alterna - brown, purple, or orange modifiers (0.4%); and, least tively as a more generalized term of colored lamellae). of all, unmodified purple (0.05%). When manufactured as cut stones, these are typically In the description of color grades, it is more difficult oriented to minimize the face-up appearance of the for the eye to interpret subtle hue differences for very lamellae, resulting in an apparent even color distribu - light-colored diamonds than it is for more saturated di - tion through the table facet. Since the lamellae are amonds (King et al., 1994; King, 2006). For the lightest along the {111} plane, these pink diamonds are gener - color grades, therefore, fewer hue names are used and ally oriented such that the table facet is {100} (J. Chap - the color description spans a wider portion of the hue man, pers. comm., 2018). Micro scopic observation of wheel. Among unmodified pink diamonds, 54% were the lamellae is best accomplished looking through Faint, Very Light, or Light in color (figure 3, right). the pavilion facets of the diamond. Below we chronicle the main causes of color in the These colored lamellae are caused by shear stress form of various structural defects, through which and natural plastic deformation (figure 5, A and B). “pink” diamonds can be divided into different groups. Deformation to form these lamellae only occurs dur -

354 NATURAL -C OLOR PINK DIAMONDS GEMS & G EMOLOGY WINTER 2018 COL OR D ISTR IBU TION

35%

N Unmodified pink diamonds O I

T 30% All others (inc luding purple- Bro wnish pink A pink, orang y pink, bro wn- R

to pinkish S U pink, etc.) T bro wn, 17% D 25%

Bro wnish orang y A N S

Pinkish purple to pink to orang y / O purplish pink, 28% pink,10% E M N 20% A O I T

Bro wn, 3% D

F " 15% K O

Purple/bro wn- N N purple/gray- I P O purple, 1% " I 10% T N I Pink, 40% U

Red/bro wn-red/orange-red, B I 5%

0.9% R T S I

D 0% Faint Very Light Fancy Fancy Fancy Fancy Fancy Fancy Light Light Intense Vivid Deep Dark

Figure 3. Color distribution of 90,000+ pink, purple, brown, orangy pink, and red diamonds graded at GIA between 2008 and 2016. Left: Unmodified pink and purplish pink diamonds dominated within this color range, while dia - monds with a brown color description were submitted in lower quantities. Right: Saturation distribution of un - modified and modified pink diamonds. A majority of the unmodified pink samples (54%) are in the Faint to Light color range. ing a diamond’s extended residency at high tempera - Howell et al., 2015). In many diamonds these defor - ture (>900°C; DeVries, 1975; Brookes and Daniel, mation lamellae are closely associated with brown, 2001), either in the mantle or during kimberlite erup - pink, or purple color; these colors can also appear tion to the earth’s surface. These deformation lamel - evenly through the entire diamond or irregularly as lae have been attributed to slip/glide planes distinct patchy areas (figure 5, C and D). The struc - (dislocation movement) and to micro-twins (Mineeva tural defects responsible for the different color contri - et al., 2009: Gaillou et al., 2010; Titkov et al., 2012; butions in pink, brown, and purple diamonds (i.e., the

Figure 4. The vast major - VIS -NI R SPECTR A ity of “pink” diamonds owe their color to the 550 nm absorption band (A– C). Spectrum A shows a N3 (415 nm) diamond with the 550

550 nm band nm band along with pro - H3 (503 nm) nounced N3 and H3 cen - ters. Spectrum D is for a A: Fancy pink brown diamond with a E

C gradually increasing ab - N B: Fancy Light purple-pink A sorption toward lower B R wavelengths that creates O S

B C: Light br own-pink the brown color; a very A shallow 550 nm absorp - tion band has little effect D: Fancy Dark br own NV – (637 nm) NV 0 (575 nm) on the color. Spectrum E E: “Golconda” Fancy Light pink is for a “Golconda pink,” which shows absorption 400 450 500 550 600 650 700 750 800 by NV 0/– centers. The spectra are offset verti - WAVELENGTH ( nm ) cally for clarity.

NATURAL -C OLOR PINK DIAMONDS GEMS & G EMOLOGY WINTER 2018 355 Slip/Glide planes Graining Uniform Patchy

A B C D

Figure 5. A: These pink type IaA>B diamonds show the color concentrated within pink lamellae (identified as “slip/glide planes” by GIA), while the remainder of the diamond is comparatively colorless. B: These type IaA

“brown absorption continuum,” the 550 nm absorp - “slip/glide planes” to distinguish them from the tion band, and NV 0/– centers) are discussed in later other type of colored lamellae. In type IaAatoms the gemological literature. around a vacancy) in the diamond (Gaillou et al., In faceted diamonds, the colored lamellae of type 2010). As diamonds reside in the mantle, or are ex - IaAtemperatures, the A-aggregates de - the apparent contrast in color within type IaA>B crease as they combine to form B-aggregates (Taylor “pink” diamonds (compare figure 5A with 5B). Re - et al., 1990). “Pink” diamonds where A-aggregates cent research (see box B) shows that these classifica - outnumber B-aggregates (i.e., type IaA>B, such as tions are not quite so distinct for diamonds that are some Siberian diamonds; Titkov et al., 2008) show low in nitrogen, and the visual distinction between colored lamellae that appear distinctly different colored “graining” and “slip/glide planes” becomes from “pink” diamonds where B-aggregates are challenging. greater than A-aggregates (i.e., type IaAB monds, and some pure type IaB diamonds, do not “pink” diamonds (designated below as Group 2), the show visible evidence of colored lamellae (either as colored lamellae appear distinct and straight along “graining” or “slip/glide planes”), and the “pink” the {111} planes, and the pink color is restricted to color is uniformly distributed. However, type IIa these colored lamellae (Gaillou et al., 2010). Be - “pink” diamonds do occasionally exhibit graining (ap - tween these lamellae, the diamond is colorless (e.g., pearing as colorless, brown, or pink) similar to that figure 5A). GIA often refers to these lamellae as seen in type IaA

356 NATURAL -C OLOR PINK DIAMONDS GEMS & G EMOLOGY WINTER 2018 Brown General Absorption. Brown color is quite additional absorption at lower wavelengths (figure 4, common in natural diamonds, and a brownish tinge spectrum A) that can alter the observed color. is reportedly observed in 98% of all as-mined dia - monds, whether or not they are deemed gem-quality Nitrogen-Vacancy Centers . “Pink” diamonds natu - (Dobrinets et al., 2013). While brown diamonds are rally colored by NV 0/– centers are extremely rare; abundant, most are either not faceted or not submit - however, these centers are almost exclusively the ted for grading reports unless there is another con - cause of color in treated and synthetic “pink” dia - tributing color such as pink, orange, or yellow or if monds. Absorption by the nitrogen-vacancy centers they can be faceted in such a way as to receive a grade (the ZPL for the neutral NV center is at 575 nm, on the D-to-Z scale (King et al., 2008). As mentioned while the negative NV center is at 637 nm) along previously, only 3% of the 90,000+ samples studied with their sidebands creates the absorption that pro - here had an unmodified brown color. duces the pale, uniform “pink” color in these very The vast majority of brown diamonds have their rare type IIa diamonds (figure 4, spectrum E). They color ascribed to vacancy clusters (i.e., clusters of va - are often referred to as “Golconda pink” diamonds cant carbon-atom positions; see Hounsome et al., (Fritsch, 1998). The 34.65 ct Fancy Intense pink Prin - 2006; Fisher et al., 2009) that are created through nat - cie is a famous example of “Golconda pink,” and it ural plastic deformation. These clusters are generally sold for more than $39 million at auction in 2013 thought to contain approximately 40–60 vacancies, (Christie’s, 2013). While the Princie originates from and the dissociation (generally through high-pressure, the historical mining region of Golconda in India, high-temperature HPHT annealing) of these vacancy today the term “Golconda” is generally associated clusters leads to the removal of brown color. These with this cause of color and not with any geographi - vacancy clusters create a so-called brown absorption cal link to the ancient mining region. continuum (Fisher et al., 2009; Dobrinets et al., 2013) that causes a gradually increasing absorption across OCCURRENCE AND FORMATION the visible spectrum toward lower wavelengths (fig - The formation mechanism for “pink” diamonds in ure 4, spectrum D). This brown absorption contin - the earth varies significantly depending on the defects uum can coexist with other color centers (e.g., figure responsible for the color. Historical sources include 4, spectrum C). India, Brazil, Indonesia (Borneo), and southern Africa. Brown coloration (by itself or in combination with But before the Argyle mine in Australia opened in other color centers) can also have other origins, in - 1983, there was no stable supply of “pink” diamonds, cluding high concentrations of isolated substitutional and many of those currently available come from Ar - nitrogen, CO , and micro-inclusions (Hainschwang et gyle (figure 6; Hofer, 1985; Shigley et al., 2001; Shirey 2 al., 2008; Dobrinets et al., 2013). However, these are and Shigley, 2013). More recently, mines in the Siber - not as commonly observed as those diamonds with ian and Arkhangelsk regions of Russia have been doc - the brown absorption continuum. umented as producing pink to purple-pink diamonds (Titkov et al., 2008; Smit and Shor, 2017). Finding new 550 nm Visible Absorption Band. The most common deposits with “pink” diamond production is impor - cause of “pink” color is a broad absorption band cen - tant to the trade, since the final year of planned oper - tered at around 550 nm (Orlov, 1977; figure 4, spec - ation at the Argyle mine is 2020 (Shor, 2018). trum B). The defect responsible for this visible absorption band has not yet been identified, but it “Pink” Diamonds with 550 nm Absorption Band. has been correlated with plastic deformation (Gaillou Occurrence. Nitrogen content and the extent of ni - et al., 2010; Howell et al., 2015). In conjunction with trogen aggregation in the atomic structure, and by in - the 550 nm band is a smaller, narrower band within ference the diamond’s geographic origin, leads to very the UV range centered at ~390 nm (Fisher et al., 2009; different observations of how “pink” color due to the Byrne et al., 2012a). 550 nm band is distributed within the diamond. In “pink” type Ia diamonds, nitrogen often creates Sources for Group 1 “pink” diamonds (type IaAVenezuela. Group 2 served color. The N3 defect (zero phonon line, or ZPL, “pink” diamonds (type IaA>B, slip/glide planes) are at 415.2 nm) and the H3 defect (ZPL at 503.2 nm) principally mined in southern Africa, Canada, and along with their vibronic structure (sidebands) create Russia (Howell et al., 2015).

NATURAL -C OLOR PINK DIAMONDS GEMS & G EMOLOGY WINTER 2018 357 or could only form, under high-pressure conditions) have been documented in GIA’s New York laboratory, but their geographic source location is unknown. One of the few superdeep IIa “pink” diamonds for which GIA does have locality information is a 23 ct rough pink type IIa diamond found at the Williamson mine (Tanzania) in November 2015 (Smith et al., 2017; Petra Diamonds, https://www.petradiamonds.com/ about-us/our-heritage). Fisher et al. (2009) also cited the Williamson mine as a source of type IIa pinks, though production numbers are unknown. The Williamson mine has also been a source of high- nitrogen type Ia “pink” diamonds (Gaillou et al., 2012). Although “pink” diamonds are known from sev - eral deposits around the world, they remain a very small percentage of the overall production from any particular deposit. The Argyle mine is well known as a source for “pink” diamonds, but these account for much less than 0.1% of the overall production, where 72% of the production is brown and 27% is colorless to yellow (Shigley et al., 2001; figure 6). At the Lomonosov deposit in northwestern Russia, only 0.04% of production is fancy-color diamonds (purple, pink, violet, green, yellow, and brown; Smit and Shor, 2017). Some parcels from the Mir deposit in Siberia contain 1–6% pink to purple diamonds, although no overall production data were available (Titkov et al., 2008 and references therein).

Formation. Pink-brown color is often associated with deformation lamellae in the diamond, and the geolog - ical process responsible for this deformation is often Figure 6. The Argyle mine is well known as a source mountain building (Stachel et al., 2018). A geologically for pink diamonds, although these account for much modern example of mountain building is the Hi - less than 0.1% of the overall production. Photo © malayan range in Asia, which is increasing in eleva - Rio Tinto. tion due to collision between the Eurasian and Indian tectonic plates. The remnants of mountain ranges that developed during collisional processes millions to bil - An abundance of locality data is available on type lions of years ago, and have since been eroded away, Ia “pinks” but not for type IIa “pink” diamonds. are called “orogens.” During continental collision, There are several observations of type IIa “pinks” that rocks become highly deformed with such severe min - coincide with Argyle “pinks” such as color range, the eral recrystallization that any diamonds and other presence of wavy graining in some type IIa diamonds, minerals in the rock can take on a “flow” texture. and some photoluminescence peaks that appear in The Argyle mine, the most famous supplier of oc - type IIa, IaB, and IaA250 km in the earth’s mantle (Smith et al., et al., 1985), and after their formation resided near 2017). Pink-brown type IIa diamonds with superdeep the base of the lithosphere (Stachel et al., 2018). It is mineral inclusions (i.e., minerals that are only stable, in this high-temperature, high-deformation environ -

358 NATURAL -C OLOR PINK DIAMONDS GEMS & G EMOLOGY WINTER 2018 ment near the convecting mantle that the pink- trapped next to isolated substitutional nitrogen. Al - brown-red color in these diamonds likely originated. though these defects are rarely preserved in nature, The Lomonosov mine, which produces pink-pur - they are relatively easy to create in a laboratory. ple-brown diamonds, also lies within an area that ex - Three rare conditions are needed to produce natural perienced ancient subduction and continental “Golconda pink” diamonds. collision—around 1.9 billion years ago—and is First, these diamonds need low nitrogen concen - known as the Lapland-Kola orogen (Daly et al., 2006). trations such that when any nitrogen present in the Although the Lomonosov diamonds have not been lattice combines with vacancies upon eventual an - dated, it is very possible that they formed in associa - nealing, no infrared-active nitrogen is detected. This tion with subduction, or experienced deformation re - would classify the diamond as rare type IIa, which lated to the subduction-collision events, during their according to a recent survey make up only 1.3% of residence at depth in the lithospheric mantle (Smit diamonds submitted to GIA for grading (out of 3.5 and Shor, 2017). million diamonds; Smith et al., 2017). Diamonds’ presence within an orogenic belt does Secondly, vacancies need to be present in the dia - not necessarily imply high deformation that could re - mond lattice. Vacancies in the lattice are created ei - sult in pink-brown colors. The Ellendale mine is in ther through irradiation or deformation. Radiation the King Leopold orogen, a geologic region that expe - exposure is rare in the mantle, and typically only oc - rienced mountain building 545 million years ago along curs once the diamond is erupted to the earth’s sur - the southwest margin of the Kimberley craton in Aus - face and has been in contact with crustal rocks or tralia. Although diamonds from this deposit are fa - fluids that contain sufficient concentrations of ele - mous for their fancy colors, they are yellow (not the ments such as uranium, thorium, or potassium pink-brown colors typically associated with deforma - (Breeding et al., 2018). Since high temperatures above tion) and the defects responsible for their yellow color 600°C are needed to combine vacancies with nitro - are the very common “cape” absorption series (N3 gen, it is more likely that deformation processes in and N2 defects). Additionally, cathodoluminescence the mantle are responsible for vacancy creation. For images of the internal growth features in these dia - example, brown coloration due to vacancy cluster monds are not dominated by deformation lamellae formation often originates from deformation of dia - (Smit, 2008). Together, the bodycolor and lack of de - mond in the mantle (Fisher et al., 2009). formation features suggest that the 1.4 billion-year- Thirdly, in order to preserve isolated nitrogen for old Ellendale diamonds survived in the lithospheric annealing, and prevent any NV centers from anneal - mantle through the Proterozoic orogenic event (until ing further to form A and B nitrogen centers, the dia- their eruption at 20 Ma) without being significantly monds must not have experienced high tem peratures deformed, implying that the King Leopold orogenic of greater than 900°C for more than a few million event was limited to the earth’s crust. years (Smit et al., 2016, 2018). The exact geological environment in the deep Pink Diamonds with NV 0/– Centers. Occurrence. Al - earth where all these conditions are met is not clear. though pink diamonds colored by NV 0/– centers (i.e., It is possible that some “Golconda pink” diamonds “Golconda pinks”) are associated with the ancient di - are superdeep, since type IIa diamonds often have a amond mining and trading center of Golconda in India sublithospheric origin (depths >300 km; Smith et al., (now Hyderabad), there are no known reliable modern 2016, 2017), and these superdeep diamonds often ex - sources in this region. Natural diamonds with suffi - hibit deformation features. For example, brittle frac - cient concentrations of NV 0/– absorption to contribute tures along the {110} crystallographic planes as well to the pink color are extremely rare. The “Golconda as polygonized dislocation networks have been ob - pink” diamonds examined in GIA laboratories (0.6% served in deformed superdeep diamonds (Smith et al., of 1,000 randomly selected “pink” diamonds; see the 2016, 2017). Deformation processes responsible for Absorption Spectroscopy section for further details) do these features could be related to mantle convection not have any geographic source information. or deep subduction processes. However, sublithos - pheric mantle temperatures are in excess of 1400°C, Formation. The combination of geologic conditions and it is not clear how isolated nitrogen and NV 0/– necessary to create and preserve NV 0/– centers in nat - would survive at these temperatures without being ural diamonds is exceedingly uncommon. NV 0/– cen - annealed to more aggregated nitrogen, which is com - ters form when vacancies in the diamond lattice are mon in superdeep nitrogen-bearing diamonds.

NATURAL -C OLOR PINK DIAMONDS GEMS & G EMOLOGY WINTER 2018 359 WEIG HT D ISTR IBU TION O F “ PIN K” DIAMOND S

14%

E Pink G

N Purple-pink A 12% R

Figure 7. Histogram

T Br own-pink

H showing the weight dis -

G Orangy pink I tribution of the 90,000+

E 10%

W Br own diamonds graded by

N

I GIA from 2008 to 2016

Purple

S 8%

D Red in the pink, purple, N orangy pink, brown, O

M 6% and red color range. A I The histogram is di - D

F vided into 0.1 carat in - O

4%

E crements between 0 G

A and 5 carats. Also T

N 2% shown are the remain - E

C ing 2% that weighed R E

P more than 5 carats. 0% 0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 >5 CARA T WEIGHT

LABORATORY GRADING While the fancy shapes are generally evenly distrib - Stones weighing less than 1.0 ct comprised 83% of uted across the range of weights and color satura - GIA intake of the 90,000+ natural “pink” diamonds tions, the rounds are predominantly seen among the in this study, and more than half (56%) weighed less smaller and lighter-color diamonds. Of all fancy- than 0.5 ct (figure 7). This chart clearly shows that color “pink” rounds, 90% are less than one carat and while the total number of “pink” diamonds appears 42% are Faint, Very Light, or Light. to be quite large—compared to 15,000+ in the blue/gray/violet group (Eaton-Magaña et al., 2018) ABSORPTION SPECTROSCOPY and 50,000+ in the green group (Breeding et al., To investigate the distribution of the major causes of 2018)—all colored diamonds submitted to GIA an - color, we studied the visible absorption and IR ab - nually represent approximately 3% of overall intake. The vast majority are also quite small. Noticeable spikes in quantity are observed near important carat- Figure 8. The shape distribution of the 90,000+ “pink” weight thresholds (1.0, 1.5, 2.0, 3.0 ct, etc.), and many diamonds graded by GIA from 2008 to 2016. have a light color (figure 3, right). This chart also serves to illustrate that while cut - ters make faceting and polishing decisions to maxi - Heart, 6% mize the face-up color and appearance, weight Oval, 9% Mar quise, 5% thresholds are often important considerations as Cushion, 13% well. Diamonds with purplish pink coloration are Squar e, 3% much more prevalent at weights below 1 carat, while Other , 2% those with brown coloration are much more preva - Emerald, 2% lent above 1 carat (figure 7), likely because it is only Rectangle, 16% worthwhile to submit brown diamonds in larger sizes. Round, 24% Although colored diamonds are often fashioned Pear , 20% as fancy shapes to enhance their face-up color, rounds represent a slight plurality (24%) of the 90,000+ “pink” diamonds (figure 8), followed by pears (20%), rectangles (16%), and cushions (13%) and others.

360 NATURAL -C OLOR PINK DIAMONDS GEMS & G EMOLOGY WINTER 2018 sorption spectra of 1,000 natural brown, pink, purple, centage with H3/N3 centers; when these were pres - and orangy pink diamonds. These were randomly se - ent, the increased absorption of blue light shifted the lected as representative of the 90,000+ samples in our perceived color to orange. However, the presence of internal GIA database. This subset of data was used N3 and H3 centers did have a dramatic effect on the to generate the charts provided in figures 9–15. tone of the color (figure 9B). In most cases, those dia - Absorption spectroscopy measurements are non - monds with faint colors only have a detectable 550 destructive and indicate the major defects and impu - nm band. For the majority of diamonds with a fancy rities within a diamond by passing light through the or deeper tone, the 550 nm band is enhanced by the stone and measuring the wavelengths (energies) ab - addition of the N3/H3 center (or the presence of both sorbed by the impurities and defects present. Defects features). that cause absorption within the visible spectrum The spectra of 50 “pink” diamonds with the 550 (between 400 and 700 nm) contribute to the body - nm band were analyzed to determine the full width color of the diamond. Other defects that are detected at half maximum (FWHM) of the band and the center only through infrared (IR) absorption spectroscopy do wavelength, as minor variations in the peak width not give information about the color-contributing de - and position of this band can affect the color. The fects but provide an indication of whether a diamond center wavelength of the absorption band is nomi - is natural, synthetic, or treated. nally at ~550 nm, but performing peak fitting using For this 1,000-diamond subset, we observed that a mixed Gaussian/Lorentzian equation shows that 992 (99.2%) contained the 550 nm band. Six were col - the center wavelength can vary from 545 nm to 565 ored by NV 0/– defects (0.6%); this percentage indicates nm. Meanwhile, the width of the absorption band the extreme rarity of “Golconda pink” diamonds even can vary from 60 nm to 100 nm. Fitting the 550 nm among this restricted population of “pink” diamonds. absorption bands from diamonds of various hues The remaining two of the 1,000 diamonds in this sub - showed that the orangy pink to pink diamonds gen - set were unmodified brown diamonds that did not erally had a lower center wavelength (545–550 nm) show an observable 550 nm band, while the other and a narrower width (60–80 nm) than purple-pink brown diamonds showed a slight 550 nm band that diamonds (555–560 nm and 7 0–100 nm, respec - did not cause sufficient absorption to affect the color tively); for example, figure 9C compares the Vis-NIR (figure 4, spectrum D). The majority of this section absorption spectra of a purple and pinkish orange di - will focus on the optical and infrared absorption spec - amond. The slight shift of the 550 nm band away tra of diamonds colored by the 550 nm absorption from the orange to red portion of the visible spec - band, and the diamonds containing NV 0/– defects will trum (for orangy pink diamonds) or away from the be briefly discussed at the end. blue (for purple-pink to purple diamonds) can con - tribute to the transmission window that leads to the Vis-NIR Absorption Spectroscopy of 550 nm “Pink” resulting bodycolor. A notable percentage of pink to Diamonds. As the vast majority of these diamonds purple-pink (~20%) also show a series of oscillations were principally colored by the 550 nm band, we overlaid on the 550 nm band at ~600 nm (figure 4, looked for subtle distinctions between the Vis-NIR spectra B and C; box B). absorption spectra to chronicle their differences. As such, the Vis-NIR absorption spectra of most dia - Fourier-Transform Infrared (FTIR) Absorption Spec - monds were distinguished by the presence or absence troscopy of 550 nm “Pink” Diamonds. Nitrogen Ag - of the N3 and H3 centers, so we could gauge whether gregation . Within the 1,000-diamond subset, 992 these had any effect on the resulting color grade. Fig - contained the 550 nm band, as stated above. Of ure 9 shows the effect of these various Vis-NIR ab - these, 243 (24%) were type IIa based on IR absorption sorption spectral features on the resulting hue and spectra along with the six diamonds colored by NV 0/– tone. Figure 9A indicates that the addition of N3 and centers (i.e., “Golconda pinks”), for a total of 249 H3 does not appear to correlate with the observed hue type IIa diamonds. The remaining 751 diamonds all (that is, the proportion of diamonds with H3 and/or contained aggregated nitrogen (figure 10A shows a N3 does not significantly change between those with variety of the IR spectra obtained for “pink” dia - brown-pink, pink, and purple-pink designations). Un - monds along with the corresponding A and B nitro - modified purple and red diamonds deviate from this gen aggregate concentrations, and figures 10B and observation and are discussed in box A. Additionally, 10C show the overall distribution of diamond type orange-pink diamonds showed a slightly higher per - along with the origin of color and color description).

NATURAL -C OLOR PINK DIAMONDS GEMS & G EMOLOGY WINTER 2018 361 BOX A: P URPLE AND RED DIAMONDS

Purple Diamonds. Unmodified purple diamonds are weak), and 33% showed yellow fluorescence (most were among the rarest of all natural diamonds (figure A-1, very weak to weak). inset). Due to their extreme rarity, none were randomly chosen for the 1,000-diamond subset. Nevertheless, we Red Diamonds. Unmodified red diamonds are some of summarize here the properties of ~50 submitted purple the most sought after gems (e.g., figure A-2, inset), and diamonds. Among these purple diamonds, 69% were for decades it was uncertain whether any diamond would less than 1 carat, and 91% were less than 2 carats. Ad - achieve this color description. To study these diamonds ditionally, 42% had a color grade of Vivid or Intense in more detail (as only five unmodified red and six with purple. orange or purple modifiers were randomly selected for the Pinkish purple to purplish pink diamonds span 1,000-diamond subset), we looked at the spectra of 100 across almost all diamond types, and about half contain Fancy red diamonds colored by the 550 nm band. Unlike sufficient quantities of H3 and/or N3 in their Vis-NIR most hue groups, red diamonds do not show any distinc - absorption spectra to affect the color (figure 9A). In con - tion in tone—a “light red” diamond would be designated trast, the vast majority (98%) of the approximately 50 as pink instead. Among these 100 diamonds, 80 were less unmodified purple diamonds did not show a significant than 1 carat and all were less than 2 carats. H3 or N3 peak (figure A-1). In these samples, more blue From examination of the Vis-NIR absorption spectra, light is then transmitted, giving rise to the purple tint. all showed very similar spectra in that they were colored Additionally, as shown in figure 9C, the purple-pink to by the 550 nm band in combination with the H3 center purple diamonds have the center of the ~550 nm band and its sideband (figure A-2). shifted slightly toward longer wavelengths, further in - Almost all of the Fancy red diamonds were type IaAB or IaA with IaA>B, and IaB. Of the type IaA150 ppm). (Very Light to Vivid) did not appear affected by nitrogen For these 100 Fancy red diamonds, all showed fluo - concentration. Instead, the color saturation appeared to rescence to long-wave UV and most (95) showed blue flu - be derived more from an increase in absorption of the orescence; the majority showed weak (30) or medium 550 nm band. (53) intensity. Of these 100, the remaining five showed The few samples deviating from the vast majority of yellow fluorescence (weak to medium). purple diamonds were a Fancy purple low-nitrogen type As the vast majority of Fancy red diamonds are type IaA

Figure A-1. These three Vis-NIR absorption spectra are from a 0.67 ct Light purple, a 0.83 ct Fancy purple, and a 0.81 ct Fancy Vivid purple. The diamond shown in the Figure A-2. The Vis-NIR absorption spectra are from three inset was identified as type IaA with 250 ppm of nitrogen Fancy red diamonds (from top: 1.01 ct, 0.48 ct, and 0.67 (Darley et al., 2011). ct) that show similar features.

VIS -NI R A BSO RPTION S PECTR A VIS -NI R A BSO RPTION S PECTR A

N3 N3 0.67 ct Light purple H4 H3 E E C C N N A A 0.83 ct Fancy purple B B H3 R R H4 O O S S B B

0.81 ct Fancy Vivid purple A A

400 450 500 550 600 650 700 750 800 850 450 500 550 600 650 700 750 800 WAVELENGTH ( nm ) WAVELENGTH ( nm )

362 NATURAL -C OLOR PINK DIAMONDS GEMS & G EMOLOGY WINTER 2018 VIS -NIR A BSO RPTION F EATUR ES IN “P IN K” D IAMOND S

100% 100% A B Optical absorption spectral fea tures 90% 90%

S S NV D 80% D 80%

N N Other

O 70% O 70% N3/H3/550 nm band M M

A A N3/550 nm band I 60% I 60% D D

H3/550 nm band F F 50% 50%

O O 550 nm band

E E

G 40% G 40% A A T T

N 30% N 30% E E C C

R 20% R 20% E E P P 10% 10%

0% 0% Bro wn Bro wn-pink Orang y pink Pink Purple-pink Red Faint Ve ry Light Fanc y Fa ncy Fanc y Fa ncy Fa ncy Fa ncy (28) (212) (63) (410) (276) (11) (89) Light (109) Light (325) Intense Vivid Deep Dark (69) (116) (166) (32) (63) (31) HUE OF DIAMOND SA TURA TION/T ONE OF DIAMOND

C

N3 (415 nm) H3 E

C (503 nm)

N 0.22 ct F anc y Vivid purple A B R

O 550 nm band S B A

1.49 ct F anc y Deep pinkish orange

400 450 500 550 600 650 700 750 800 WAVELENGTH ( nm )

Figure 9. Natural “pink” diamonds submitted to GIA over the last decade are predominantly colored by the 550 nm band. A: When a subset of 1,000 diamonds are analyzed by hue, they show minor differences in the presence/absence of H3/N3 centers, indicating that intensity differences in peak characteristics, and not the pres - ence/absence of these features, generally create the differences in hue. B: The addition of the N3 and/or H3 centers leads to an increase in overall absorption, creating more saturated colors. C: A comparison of the Vis-NIR absorp - tion spectra between a 0.22 ct Fancy Vivid purple and a 1.49 ct Fancy Deep pinkish orange diamond. While both have a ~550 nm absorption band, differences in this band and the presence of other centers lead to vastly different colors. The pinkish orange diamond shows pronounced H3 and N3 centers that are lacking in the purple diamond. For the pinkish orange diamond, the 550 nm band is centered at 548 nm with a width of 62 nm. For the purple dia - mond, the center is located at 556 nm and the peak width is much wider, 105 nm. The vertical line indicates the center point for each of these bands.

Those 170 termed as type Ia (17%) had saturated ni - Figure 10B shows that the type IIa “pink” dia - trogen concentrations in their IR absorption spectra, monds were overwhelmingly colored by the 550 nm and the specific A and B nitrogen aggregates were too band alone and generally did not include the addi - great to be resolved with our instrumentation and tional H3 and N3 centers. This is not surprising, as thus could not be calculated. The remaining 581 di - type IIa diamonds do not have sufficient nitrogen amonds had sufficiently low nitrogen to determine concentration to generate nitrogen-related defects the aggregate concentrations (Boyd et al., 1994, 1995). that could be detected by visible absorption and af - Of those, 396 were type IaAB (13%), 31 pure type IaA (3%), and 29 pure Among the diamonds with nitrogen aggregates, type IaB (3%). many were colored by H3 and/or N3 that appeared

NATURAL -C OLOR PINK DIAMONDS GEMS & G EMOLOGY WINTER 2018 363 IR S PECTR A A ND DIAMOND T YPE D ISTR IBU TION

Intrinsic diamond Nitrogen A fea tures aggrega tes

3107 Fanc y Intense purple-pink; sa tura ted nitrogen Type Ia E

C Fanc y pink; A = 83 ppm; B = 238 ppm

N Type IaA

O Fanc y purple-pink; A = 220 ppm; B = 26 ppm S

B Type IaA>B A Fanc y pink; A = 20 ppm; B = 46 ppm Type IaA

5000 4000 3000 2000 1000 WAVENUMBER ( cm –1 )

400 100% BCOther Red 90% 350 NV Purple-pink S D S N3/H3/550 nm band 80% Pink N D 300 N3/550 nm band Orang y pink O N 70% M O H3/550 nm band Bro wn-pink A M

250 I 550 nm band 60% Bro wn A D I

F D 200 50% O F

E O

G 40% R 150 A E T B

N 30% M 100 E U C

R 20% N E

50 P 10%

0 0% Ia IaA IaA>B IaAB IaA

364 NATURAL -C OLOR PINK DIAMONDS GEMS & G EMOLOGY WINTER 2018 DISTR IBU TION O F A - AND B -A GGREGAT ES

600 200

Kno wn Arg yle diamonds B B = = Kno wn Siberian diamonds A A Type IaA

S S E E T T Type IaA>B A 300 A 100 G G E E R R G G G 200 G A A - - B B 50

100

0 0 0 100 200 300 400 500 600 0 50 100 150 200

A-A GGREGA TES ( ppm ) A-A GGREGA TES ( ppm )

Figure 11. The A and B nitrogen aggregates for 581 type IaAB diamonds are plotted against each other along with A/B concentrations for 62 known Argyle diamonds and a pair of known Siberian diamonds (most IR spectra of the known Siberian samples had saturated spectra, thus reducing number of plottable datapoints). Also shown is a di - agonal line designating where A- and B-aggregates are equal and a green slope line in the inset showing where the vast majority of data are clustered.

Taylor, 1997). Even if the total nitrogen concentra - strongly suggests that all of these diamonds experi - tion is different between diamonds, the transition enced similar time/temperature conditions within the from A-aggregates to B-aggregates increases at higher earth and potentially from the same mine. The sheer temperatures and longer timescales. If a population quantity of diamonds plotted here (compared with of diamonds has similar A-to-B ratios, they likely those scattered among the rest of the graph) imply that resided under similar time and temperature condi - they come from the most abundant source—the Ar - tions, increasing the likelihood that they originated gyle mine—although they could also be from Namibia from the same source. or Venezuela. Also plotted are the data of 67 dia - As mentioned previously, 581 of the “pink” dia - monds, unrelated to this study, that are known to be monds from the 1,000-diamond subset had IR spectra sourced from the Argyle mine (unpublished GIA data, from which the concentrations of A- and B-aggre - 2007). In contrast, pink to purple diamonds sourced gates could be calculated (in atomic parts per million, from Siberia are predominantly type IaA>B, and two or ppma; figure 11). These were calculated individu - known Siberian samples with unsaturated IR spectra ally by baseline correcting each spectrum, and then (Titkov et al., 2008) are also shown alongside the data calculating A- and B-aggregates using a spreadsheet from this study. This geographical distinction based provided by Dr. David Fisher (DTC Research Center, on nitrogen aggregation has been documented before Maidenhead, UK) using absorption coefficients from (Gaillou et al., 2010; Howell et al., 2015), and a good Boyd et al. (1994, 1995). The majority of the 581 dia - hypothesis of origin is possible, though not with any monds contained more aggregated nitrogen as B cen - reliable certainty, as “pink” diamonds originate from ters (type IaA

NATURAL -C OLOR PINK DIAMONDS GEMS & G EMOLOGY WINTER 2018 365 BOX B: PL F EATURES OF PINK TYPE II A TO TYPE IAAB D IAMONDS

Photoluminescence Band Associated with 550 nm Ab - 2. After exposure to the high-energy UV of the sorption Band. As part of our identification procedure Diamond View, the intensity of the emission band for pink diamonds, GIA routinely collects their PL spec - decreases and shows a bleaching behavior similar tra (e.g., using 514 nm excitation). One interesting and to that of the 550 nm absorption band. When the consistent feature in the PL spectra of these pink dia - pink diamond returns to its stable color, so does monds is a wide emission band extending from ~600 to the intensity of the emission band. 750 nm, with a series of smaller oscillations overlaid on 3. In diamonds with pink and brown graining, the in - the larger emission band (e.g., the black trace in figure tensity of the emission band is lower in the brown B-1). This emission band is consistently seen in dia - graining than in the pink graining. monds colored by the 550 nm absorption band; the ab - 4. This emission band is absent from diamonds that sorption band often, but not always, shows similar do not show a 550 nm absorption band. oscillations at ~600 nm (e.g., the red trace in figure B-1; see also figure 4, B and C). The oscillations in the 550 5. This emission band is similar to one previously doc - nm absorption band are most often detected in saturated umented by Gaillou et al. (2010), appearing only pink to purple-pink diamonds. While this feature is only within photoluminescence spectra obtained from rarely detected in light pink diamonds with a weak 550 pink lamellae and absent from colorless portions. nm absorption band, the 600–750 emission band may Photoluminescence Maps of Type IIa and Type IaAB Di - still be detected in their PL spectra. Although this emis - amonds. We wished to study this emission band in more sion band appears qualitatively similar to the PL band detail, and therefore 455 and 532 nm photoluminescence associated with the 480 nm band (figure 21C), the two maps were analyzed to examine several features in pink are dissimilar in overall band width and the spacing be - diamonds. These included H3, H4, the 600 –750 nm tween the oscillations. emission band, and several other common features seen Additional interesting observations about this 600– in PL spectra (e.g., figure 14). These results will be dis - 750 emission band include: cussed in an upcoming paper. 1. The spacing of the oscillations between the 550 Gaillou et al. (2010) performed PL spectra line scans nm absorption band and the emission band show across a series of flat plates containing pink lamellae. In a pronounced similarity. addition to finding this 600–750 nm emission band

VIS -NI R A BSORPTION A ND PL S PECTR A

E Figure B-1. The Vis-NIR C m n

N absorption spectrum (red 0 m A 2 n

B trace) for a purple-pink di - 4 R 1

O amond is shown along m n S

7 B with its 514 nm PL spec - A trum (black trace). Both Vis-NIR absorption spectrum of type IIa Fancy purple-pink diamond spectra were collected at liquid nitrogen tempera - ture and show an interest - ing symmetry in the P

514 nm PL spectrum of same diamond L

oscillations observed I N

7 within the 550 nm absorp - T

GR1

2nd or der n 1 E m 4 Raman N tion band and the emis -

Raman n S m 2 I 0 sion band. T

n m Y

500 550 600 650 700 750 800 850 900 WAVELENGTH ( nm )

366 NATURAL -C OLOR PINK DIAMONDS GEMS & G EMOLOGY WINTER 2018 High-nitr ogen type IaA>B pink diamond PL map of 600–750 nm emission band PL map of H3 peak width (nm) with visible slip/glide planes (normalized using ar ea of diamond Raman peak)

100 1.5

0 0.5

Figure B-2. Photoluminescence maps of the 600–750 nm emission band and H3 peak width show excellent agree- ment with the visual observation of pink color along deformation lamellae in this type IaA>B pink diamond. The intensity bars show the range of values.

solely within the pink lamellae, they observed that the bution of the 600–750 nm emission band is consistent Raman width noticeably increased as well. They were with its color distribution, with wavy-appearing concen - unable to produce the PL maps that are possible with trations of color that are slightly more saturated than the today’s technology, and they did not have type IIa pink surrounding diamond. For higher-nitrogen type IaA>B di - diamonds available for comparison. amonds (i.e., Group 2 pink diamonds) that have pro - As PL is a very sensitive method capable of collecting nounced slip/glide planes, there is a distinct change in information on defects at the parts-per-billion (ppb) level, spectral properties within the pink slip/glide planes (fig - it can detect subtle variations in diamond far better than ure B-2); these changes include a pronounced increased the eye can distinguish pink color. Additionally, we are un - in the 600–750 nm emission band, along with other fea - able to perform visible absorption spatial mapping on these tures such as H3 peak width. In contrast, the PL mapping pink diamonds, so we cannot map the distribution of the of type IIa diamonds showed that the spatial distribution 550 nm band in pink diamonds. We can collect PL spectral of the 600–750 nm emission is nearly uniformly distrib - maps at liquid nitrogen temperature, however. Based on uted, as is the distribution of the pink color. the observations of this 600–750 nm emission band, it is Low-nitrogen “pink” diamonds generally show a monitored as a proxy of the 550 nm absorption band. mixture of spectral features, with some exhibiting a Here we performed PL mapping on type IIa and type combination of traits seen in the uniform color distribu - IaAB diamonds. In type IaAB pink diamonds, these data tion of type IIa pinks, the irregular graining seen in type will show the spectral differences between the colored IaAB (i.e., Group 2). Diamonds as spatially uniform as their pink color. We paid particular with low nitrogen concentrations generally show a com - attention to low-nitrogen type Ia diamonds to examine bination of features seen in type IIa and Group 1 dia - the progression from the homogeneity of type IIa dia - monds such as a uniform pink color distribution, though monds to the color segregations seen in type Ia diamonds. there are some instances of visible pink graining or a uni - For moderate-nitrogen type IaA

NATURAL -C OLOR PINK DIAMONDS GEMS & G EMOLOGY WINTER 2018 367 and the other types, including the saturated type Ia increases the overall absorption in these diamonds, diamonds, are sourced from other mines, then 425 of leading to more saturated colors. the 992 diamonds with the 550 nm band, a total of at least 43%, originated from Argyle. Other IR Features. Of the 1,000-diamond subset, half The nitrogen aggregation results for these dia - were randomly selected and surveyed for the pres - ence in their spectra of the N VH defect (3107 cm –1 monds, shown in figure 11, are also split according 3 to the color description of the “pink” diamond. This center), the platelet feature, and “amber” centers. indicates several interesting trends. Among the type The 3107 cm –1 peak was designated for many IaAB diamonds (i.e., the right side type IaA>B and IaAB data of all 581 “pink” diamonds al., 2017). None of the type IIa and IaB stones in our shown in figure 11 by the total aggregate concentra - dataset had a platelet peak present in the IR spec - tion into five different ranges. The concentration trum, which is to be expected due to their lack of A range of each group was determined in order to create centers (Humble, 1982). The IaA, IaA>B, IaAB that appears due to the higher incidence cm –1 . It has been suggested that the “amber” centers of the N3 and H3 centers (figure 12C and consistent are defective A-aggregates formed along plastic de - with figure 9). For the highest range shown in figure formation planes (Massi et al., 2005). Two specific 12C (>250 ppm total nitrogen), more than 80% of the bands were surveyed for this study: the “amber” cen - type IaA

368 NATURAL -C OLOR PINK DIAMONDS GEMS & G EMOLOGY WINTER 2018 HUE: AB 100% 100%

80% 80%

A 60% 60% 40% 40%

20% 20%

0% 0% 0–40 ppm 40–70 ppm 70–150 ppm 150–250 ppm > 250 ppm 0–40 ppm 40–70 ppm 70–150 ppm 150–250 ppm > 250 ppm

Brown Brown-pink Orangy pink Pink Purple-pink Brown Brown-pink Orangy pink Pink Purple-pink

TONE/ SATURA TION: AB 100% 100%

80% 80%

B 60% 60% 40% 40%

20% 20%

0% 0% 0–40 ppm 40–70 ppm 70–150 ppm 150–250 ppm > 250 ppm 0–40 ppm 40–70 ppm 70–150 ppm 150–250 ppm > 250 ppm

Faint to Fancy Light Fancy Fancy Intense to Vivid Fancy Deep to Dark Faint to Fancy Light Fancy Fancy Intense to Vivid Fancy Deep to Dark

VIS- NIR ABSORPTION FEA TURES: AB 100% 100%

80% 80%

60% 60% C 40% 40%

20% 20%

0% 0% 0–40 ppm 40–70 ppm 70–150 ppm 150–250 ppm > 250 ppm 0–40 ppm 40–70 ppm 70–150 ppm 150–250 ppm > 250 ppm

550 nm band H3/550 N3/550 N3/H3/550 550 nm band H3/550 N3/550 N3/H3/550

Figure 12. Type IaAB diamonds were split into two groups, IaAB, and sorted based on total nitrogen concentration. A: The effect of total nitrogen concentration on hue. B: The effect of increasing nitrogen on the tone of the diamond. C: The effect of total nitrogen concentration on features present in Vis-NIR absorption spectra.

while the 4165 cm –1 “amber” center can survive ter being a defective A center (Massi et al., 2005). As heating until 1900°C (Eaton-Magaña et al., 2017). As the concentration of A-aggregates increased, the like - these two features are stable to different tempera - lihood of one or both “amber” peaks increased as tures, they are likely different centers. Previous re - well; the type Ia diamonds showed a high percentage ports have shown that the 4065 cm –1 “amber” center of “amber” centers, likely corresponding to the high is more prevalent in diamonds of type IaA>B (Gaillou concentration of A-aggregates within them (although et al., 2010). the saturated spectra do not allow us to accurately The IR spectra were categorized as having neither make that determination). peak, one of these peaks, or both (figure 13). None of the type IaB stones and very few (14%) of the type IIa Diamonds Colored by NV 0/– Centers. Only six of the diamonds showed either the 4065 or the 4165 cm –1 1,000-diamond subset were colored by NV 0/– centers, peak, consistent with the model of the “amber” cen - thus rendering a very small sample size and provide

NATURAL -C OLOR PINK DIAMONDS GEMS & G EMOLOGY WINTER 2018 369 AMBER C EN TE R O CCURR ENCE

100%

90%

80% S D

N 70% O

M Figure 13. IR absorption

A 60% I peaks related to D

F “amber” centers are 50% O

E plotted based on dia - G

A 40% mond type. The number T

N of samples tested within E

C 30% each diamond type is R E

P shown in parentheses. 20%

10%

0% IIa (128) IaB (15) IaAB (57) IaA (18) Ia (75)

DIAMOND TYPE

Neither Only 4065 cm –1 Only 4165 cm –1 Both

little more to illuminate beyond what has already sorption band often show features that either contain been discussed in the Causes of Color section. As ex - vacancies or correlate with plastic deformation. For pected, all six showed type IIa IR absorption spectra example, the 566 nm triplet is seen in pink to brown that were featureless except for intrinsic diamond diamonds of both type Ia and IIa (i.e., those with and features. In the Vis-NIR absorption spectra, the six without colored graining; GIA unpublished data). diamonds (three orangy pink, two pink, and one Common vacancy-related peaks include the H3 brownish pink) had NV 0/– centers at 575 and 637 nm (NVN 0 at 503.2 nm), H4 (NVN – at 496 nm), GR1 (V 0 along with their accompanying sidebands, with the at 741.2 nm), and NV centers (NV 0 at 575 nm, NV – most prominent of those centered at ~520 and 620 at 637 nm; figure 14). The NV 0/– centers are often nm (figure 4, spectrum E). Of these, four had de - present in diamonds containing the 550 nm band but tectable GR1 peaks and four showed the 595 nm cen - not in sufficient concentrations to be detected in the ter commonly seen in irradiated diamonds (Breeding Vis-NIR absorption spectra, and therefore they do not et al., 2018). The presence or absence of these radia - affect the color. Peaks at 657, 661, and 668 nm are tion-related peaks did not correspond with the graded often seen together in type IIa, IaAlasers of different wave - and slip/glide planes that are observed in type IaAB lengths to produce emission spectra that reveal the diamonds. optical defects present in a diamond. PL analysis is The peak width of the diamond Raman line is af - one of the most useful and sensitive techniques for fected by the local strain environment, and an in - defect characterization. Using PL spectroscopy, de - crease in crystallographic strain leads to a broadening fects occurring at parts per billion (ppb) concentra - of the peak width (Grimsditch et al., 1978). The dia - tions (and therefore not necessarily contributing to mond Raman line has been shown to increase in the diamond’s bodycolor) can easily be detected width on pink grain lines in type Ia diamonds, (Eaton-Magaña and Breeding, 2016). demonstrating that the pink lamellae lie in planes of plastic deformation (Gaillou et al., 2010). Diamonds Containing the 550 nm Absorption Band. In contrast, type IIa “pink” diamonds rarely show The PL spectra of diamonds colored by the 550 nm ab - colored lamellae and instead are evenly colored. In

370 NATURAL -C OLOR PINK DIAMONDS GEMS & G EMOLOGY WINTER 2018 PL S PECTRU M

552 nm saturated diamond Raman line 741 nm 20000 GR1 Figure 14. A typical 514

) 2nd or der S nm photoluminescence

T diamond Raman

N spectrum from a 550

U 15000

O band pink type IIa dia - C ( 637 nm mond. The 566 nm Y – T 575 nm NV 0/– I 657 nm triplet, NV centers,

S 0 10000 NV N 657 nm peak, and GR1 E T are all shown. The dia -

N 566 nm I

L mond Raman line at P 5000 552 nm is saturated.

0 550 600 650 700 750 WAVELENGTH ( nm ) type IIa diamonds showing a uniform pink col - corresponds to an increase in pink color saturation oration, this relationship of strain versus pink color in type IIa diamonds, despite the lack of easily dis - is not so easily visualized. Therefore, we plotted the tinguishable strained regions. GR1 width (i.e., increasing strain) of 239 type IIa “pink” diamonds from the 1,000-diamond subset Diamonds Colored by NV 0/– Centers. Diamonds col - against pink color intensity (figure 15). This plot ored by NV 0/– centers (i.e., those with NV 0/– centers shows that in type IIa diamonds, an increase in strain in sufficient concentrations to be detected by Vis- (represented by the peak width of the GR1 center) NIR absorption spectroscopy) will have high-inten - sity NV 0/– peaks in their PL spectra (figure 16). Due to this high intensity of the NV 0/– centers and their Figure 15. Among type IIa “pink” diamonds, the color associated vibronic structures, it can be difficult to is generally uniform rather than concentrated within detect other peaks that may be present at lower PL graining. We see that among type IIa “pinks,” an in - intensities. A peak at 561 nm is commonly seen, but crease in GR1 full width at half maximum (function - ing as a proxy for the strain measurement) corresponds with more saturated pink color. The individual points Figure 16. A typical 514 nm photoluminescence are shown along with the average values. spectrum from a “Golconda pink” diamond. The NV 0/– centers (and their vibronic structure/side - COL OR I NTEN SITY V S. G R1 W IDT H bands) and the diamond Raman line (R) are the only features detected. 2.5 Individual V alues Average ) 2.0 PL S PECTRU M m m (

) H S T 1.5 T D N 60000 I U W

O K

C 637 nm

1.0 ( A 575 nm –

E NV

Y 40000 0 P T

NV I 1 S R

0.5 N G E

T 20000 N

I Raman

L

0 P Faint Ver y Light Fa ncy Fa ncy Fanc y Intense 0 to Light Light to Vivid 550 600 650 700 750 TONE OF DIAMOND WAVELENGTH ( nm )

NATURAL -C OLOR PINK DIAMONDS GEMS & G EMOLOGY WINTER 2018 371 the structure of this defect is still unknown. Com - A mon vacancy-related defects seen in diamonds with NV 0/– centers include H3 (NVN 0), H4 (4N-2V), and GR1 (V 0). Brown graining GEMOLOGICAL OBSERVATIONS Physical Characteristics. Pink and brown color is often associated with plastic deformation lamellae Pink graining (“graining” or “slip/glide planes”). These are often observed as visible lines on the polished surface of B the diamond, known as surface graining, which re - sult from differential polishing rates along the defor - mation lamellae relative to the surrounding diamond. This leaves visible lines on the surface; un - like polishing features, these can cross multiple facets (figure 18, right). The graining usually has a hue similar to the dia - mond’s bodycolor, though in some cases diamond can exhibit both pink and brown graining (figure C 17A). Additionally, graining and slip/glide planes can appear more pronounced when the diamond is put between crossed polarizers to show the strain (i.e., anomalous birefringence; figure 17B). The disruption in the strain pattern apparent at a slip/glide plane provides further evidence that these planes are a dis - ruption of the diamond lattice (figure 17C). Figure 17. A: These two pink diamonds are rare exam - Trigons can be etched where deformation lamellae ples exhibiting both pink and brown graining. B: This intersect the surface of a rough diamond (figure 18, diamond shows slip/glide planes that are readily left). Where two of these deformation lamellae inter - viewed through crossed polarizers (right), indicating sect, it is common to see an etch channel or thin nee - that the lattice has been strongly distorted due to dle-like structure (Wang et al., 2006). Diamonds are plastic deformation. C: This 5.09 ct Fancy pink-purple etched by fluids during their mantle residency or dur - sample with obvious slip/glide planes (left) will show, ing kimberlite ascent (Fedortchouk et al., 2005; Fe - when viewed at the proper angle and with proper illu - dortchouk and Zhang, 2011), and these fluids first mination (right), that light is reflected from plastic deformation lamellae, indicating a physical disconti - exploit preexisting weaknesses in a diamond. Hollow nuity or a damaged zone of the diamond’s lattice channels known as Rose channels can also occur along structure. Photomicrographs by GIA staff. intersecting twin lamellae (Schoor et al., 2016; figure 18, right). First described by Gustav Rose in 1868, these are formed by the intersection of twin lamellae, where the twinning action can result in a straight, hollow medium (53%) to strong (25%) intensity and had a di - channel with a prismatic opening (Rose, 1868). amond type of IaAB (36%). Those with fluo - domly selected for detailed spectroscopic analyses, rescence reported as none generally were either 786 had observations of long-wave (365 nm excitation) saturated type Ia (67%) or IaA>B (29%). and short-wave UV (254 nm excitation) reported in the database. Among those with fluorescence observa - NV 0/– Centers. The rare “Golconda pink” diamonds tions, 571 (73%) showed blue fluorescence to long- (six out of the subset of 1,000 samples, or 0.6%) all wave excitation; 89 (11%) exhibited yellow showed yellow-orange-red fluorescence colors caused fluorescence and 113 (14%) no fluorescence. Those by NV 0/– centers. Although all colored diamonds with reported blue fluorescence generally showed should be submitted to a laboratory to determine the

372 NATURAL -C OLOR PINK DIAMONDS GEMS & G EMOLOGY WINTER 2018 Deformation lamellae

Rose channel

Figure 18. Left: Etching as trigons along plastic deformation lamellae in Light purplish pink type Ia diamonds, cap - tured in differential interference contrast lighting to help improve image contrast. Photomicrograph by Nathan Renfro; field of view 1.24 mm. Right: Hollow channels (called Rose channels and indicated by the red arrow) have developed at the intersections of deformation lamellae in a 0.98 ct Fancy brownish pink diamond. Photomicro - graph by Wuyi Wang; field of view 1.30 mm.

origin of their color, any “pink” diamond with yel - grading. Any “pink” diamonds that require Diamond - low-orange-red fluorescence colors should be given View imaging are returned to their stable color before special attention. leaving the laboratory. Figure 19 shows the photos and spectra of a type Bleaching. Many type IIa diamonds, including those IIa Fancy Intense pink diamond that was exposed to colored by the 550 nm band and “Golconda pink” di - amonds, change color after exposure to UV light. Al - though typically observed following exposure to the Figure 19. This 9.35 ct Fancy Intense pink type IIa di - high-energy light of the DiamondView instrument amond showed a dramatic color change to Fancy In - (with wavelengths <225 nm), bleaching can be ob - tense brownish yellow following exposure to the served following exposure to longer-wave UV but the high-energy UV of the DiamondView. The UV expo - effect is lessened at lower-energy, higher wavelengths sure caused a decrease in the ~390 and ~550 nm (Fisher et al., 2009; Byrne et al., 2012b, 2014). After bands, and the original color was completely restored exposure to UV, “pink” diamonds can change color after 30 minutes of exposure to bright visible light. to yellow, brown, or near-colorless, and these changes are due to several charge-transfer processes UV-V IS -NI R A BSO RPTION S PECTR A that have been associated with the vacancy clusters present in brown diamonds (Byrne et al., 2014). 390 nm Bleaching has also been observed during polishing of band pink diamonds (Chapman, 2014). N3 The diamond’s color always reverts back to E C H3

“pink,” though this can take anywhere from several N

A Stable color: minutes to more than a day depending on the color B 550 nm

R Befor e UV bleaching band and intensity of the light (Byrne et al., 2014). Expo - O S GR1 B

sure to bright light (such as placing the diamond in A a microscope light well) accelerates the process. As a precaution, GIA gemologists generally do not col - After UV bleaching lect DiamondView images on “pink” diamonds un - 350 400 450 500 550 600 650 700 750 800 less other data indicate potential treatment, and WAVELENGTH ( nm ) Diamond View images are always collected after color

NATURAL -C OLOR PINK DIAMONDS GEMS & G EMOLOGY WINTER 2018 373 VIS -NIR A BSORPTION S PECTRUM high-energy UV in the DiamondView. Its color dra - matically and temporarily changed to Fancy Intense brownish yellow, which was accompanied by a de - crease in the 390 nm and 550 nm absorption bands, E while the intensities of the other centers (e.g., N3, C N A

H3, and GR1) remain unchanged. B R O S B IDENTIFICATION CONCERNS A Type Ia “Pinks.” There is no known method of treat - ment to create the 550 nm absorption band in the 400 450 500 550 600 650 700 750 800 850 laboratory. Except for the rare type IaB diamonds dis - cussed below, the majority of type Ia diamonds can - WAVELENGTH ( nm ) not be treated to appear pink. Additionally, the colored graining common in type Ia “pinks” cannot PL S PECTRUM

be artificially created. ) 140000 S T

N 120000

Type IIa and Type IaB “Pinks.” As with the high-pres - U

O 100000 C

sure, high-temperature (HPHT) decolorizing treat - ( 80000 Y

ment, type IIa and type IaB pinkish brown diamonds T I

S 60000

can be HPHT-treated to remove the brown compo - N E nent (Hainschwang et al., 2003; Fisher et al., 2009). If T 40000 N I 20000 the 550 nm band is present in these diamonds, the L P pink color will be accentuated by HPHT treatment, 0 as the brown color is reduced. Diamonds that are 500 550 600 650 700 750 800 850 HPHT-treated to pink are generally subjected to lower WAVELENGTH ( nm ) temperatures (1700–1800°C) than those that are HPHT-treated to colorless (2000–2300°C; Fisher et al., Figure 20. This 5.71 ct Fancy red-brown diamond de - 2009; Dobrinets et al., 2013). As with colorless dia - rives its color from the 480 nm band, as shown by its monds, these “pink” diamonds should be assessed by Vis-NIR absorption spectrum (top). The presence of photoluminescence spectroscopy, which shows emis - the sometimes subtle 480 nm band is unambiguously identified by a distinctive emission band determined sion peaks that reliably distinguish between natural by PL spectroscopy (bottom), such as this spectrum and treated diamonds. Additionally, a few “pink” collected with 488 nm excitation. CVD diamonds appear pink due to an absorption band at ~520 nm (Wang et al., 2007); therefore, careful as - sessment is needed to verify that the color-causing vis - ible absorption band in type IIa “pink” diamonds is ment of absorption, luminescence (such as mapping centered at ~550 nm. The cause of the ~520 nm band the spatial distribution of defects; D’Haenens-Johans - in CVDs is also unknown. son et al., 2017), and gemological data should always be collected and carefully evaluated. Type IIa “Golconda Pinks.” Any diamond colored by NV 0/– centers must be examined extremely carefully. UNUSUAL EXAMPLES Both treated naturals and treated synthetics are al - A small number of brown-red to red-brown diamonds tered to a pink color by generating NV 0/– centers. owe their color to a combination of a slight 550 nm Eaton-Magaña and Shigley (2016) provide a compar - band with a 480 nm band (figure 20). The defect re - ison table contrasting the distinguishing characteris - sponsible for the ~480 nm absorption band also has tics of natural, treated, and synthetic “pink” a luminescence band centered at ~700 nm that is ex - diamonds that are colored by NV 0/– centers. Natural tremely broad with a unique wavy shape (figure 20, “Golconda pink” diamonds typically have pale col - bottom). The combination of these two broad bands ors, while treated “pink” diamonds have much more (the 480 and 550 nm absorption bands) leads to saturated colors. Although the separation of syn - strong absorption from the orange to blue color re - thetic diamonds is generally less complicated than gions. This combination of absorption features not the separation of treated diamonds, a full comple - only induced the common brown color but also cre -

374 NATURAL -C OLOR PINK DIAMONDS GEMS & G EMOLOGY WINTER 2018 Figure 21. These diamonds are from the 2016 Argyle Pink Tender in New York City. Left to right: 0.64 ct Fancy Deep pink oval, 0.75 ct Fancy Intense purplish pink trilliant, 0.91 ct Fancy Vivid purplish pink oval, 1.30 ct Fancy Intense pink heart, 1.35 ct Fancy Intense purplish pink cushion, 0.80 ct Fancy Vivid pink pear, and 0.45 ct Fancy Vivid purplish pink emerald cut. Photo by Robert Weldon; courtesy of Argyle Pink Diamonds.

ated a “transparent window” in the red region. The about type IIa “pinks,” and we could find no studies result is a red-brown coloration. of a mine that regularly produces type IIa “pinks.” Therefore, one enduring question is the source lo - CONCLUSIONS cality of the type IIa “pink” diamonds colored by Most colored diamonds receive the ingredients for the 550 nm absorption band. Of the 992 diamonds color deep within the earth as they are created—ni - colored by the 550 nm band that were randomly se - trogen, boron, nickel, and hydrogen impurities pro - lected and studied in detail, almost one-fourth (243) duce yellow, yellowish green, violet, and blue colors, were type IIa. Although this is a comparatively high while abundant micro-inclusions can create white percentage, little has been written about their and black colors. After reaching the earth’s surface, source localities (e.g., Fisher et al., 2009; Smith et diamonds can be exposed to radiation to create blue al., 2017). to green colors. However, the vast majority of natural Today, the Argyle mine produces a sizeable per - “pink” diamonds colored by the 550 nm absorption centage of the world’s “pink” diamonds (e.g., figure band are produced through deformation processes 21), although pink to purple diamonds also come deep within the earth. from countries such as Brazil, South Africa, and Rus - Many source locality reports have been written sia. Since the Argyle mine is set to close within the about type Ia “pink” diamonds from Australia and next few years, the supply of “pink” diamonds, par - Russia (Iakoubovskii and Adriaenssens, 2002; ticularly those with saturated colors or those with Titkov et al., 2008, 2012; Gaillou et al., 2010, 2012; red color descriptions, will decline unless the supply Howell et al., 2015), yet very little has been written from other producers becomes more substantial.

ABOUT THE AUTHORS ACKNOWLEDGMENTS Dr. Eaton-Magaña and Dr. Breeding are senior research scien - Discussions with GIA’s Dr. Evan Smith and John King contributed tists, Mr. Ardon is a research associate, and Dr. Shigley is a distin - to some ideas presented here. John Chapman (Gemetrix, Perth, guished research fellow, at GIA in Carlsbad, California. Dr. Smit is Australia) and Sergey Titkov provided helpful insight. a research scientist at GIA in New York.

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